The image-forming device performs image formation by supplying toner to an image-carrying member to form an image thereon and subsequently transferring the toner image onto a recording paper. In order to accurately adjust the darkness of the image formed on the recording paper, the image-forming device has a toner quantity detecting device (density sensor) for detecting the quantity (density) of toner deposited on the image-carrying member. Japanese patent application publication No. 2004-101687 discloses an image-forming device comprising such a toner quantity detecting device.
The density sensor includes a light-emitting element for irradiating a detection light beam onto the image-carrying member, and a light-receiving element for receiving the detection light beam reflected off the image-carrying member and for generating an output. The density sensor detects the density of the toner based on the output from the light-receiving element. The density sensor includes a shutter for covering and exposing the front surfaces of the light-emitting element and light-receiving element. A reflecting reference member is provided on the shutter.
Before detecting the density of toner, the density sensor irradiates the detection light beam from the light-emitting element when the shutter is closed, and sets the resultant output from the light-receiving element as an initial detection value when the light-receiving element receives the detection light beam reflected off the reflecting reference member of the shutter. Hence, even if the performance of the light-emitting element and light-receiving element degrades due to scattered toner, the density sensor maintains the same initial detection state by detecting density after adjusting the sensor output.
Here, in order to control light emission at a uniform intensity, the conventional image-forming device regulates the amount of light emission for each printing operation so that the amount of light reflected off the reflecting reference member when the shutter is in the shielding position is constant. However, in reality, individual sensors have a different response (sensitivity) with respect to the distance from the sensor to the measuring surface. Therefore, regulating the amount of light emission to achieve a constant intensity at the shutter reference position does not guarantee that the amount of light received will be constant, since the actual distance to the image-carrying member on which the patches are measured differs from the distance at which the reflected light is measured.
FIG. 1 shows an output characteristics from first and second density sensors when a reflecting reference member having certain reflection characteristics is positioned a distance of 2.5-5.5 mm from the sensors. The vertical axis in FIG. 1 represents the output from the density sensors, while the horizontal axis represents the distance (mm) between the density sensor and the image-carrying member. Further, the thin solid line in the drawing indicates the detection characteristics of the first density sensor, the bold solid line the detection characteristics of the second density sensor, and the dotted line the detection characteristics of the second density sensor adjusted based on the amount of detection light reflected off the reflecting reference member. As can be seen in FIG. 1, when the reflecting reference member is arranged at a belt position of 5 mm from the first and second density sensors, even when the light emission intensity of the first and second density sensors (solid line and bold line in FIG. 1) is adjusted to achieve a sensor output of 1.5 V for the reflection characteristics of the reflecting reference member. However, the sensor output for other distances may deviate among the two sensors.
On the other hand, when the light emission intensity is adjusted so that the output from both the first and second density sensors is 1.6 V for a shutter position 3 mm from the sensors, as indicated by the dotted line, the second density sensor detects a lower intensity of received light at the belt position of 5 mm than the first density sensor.
The irregularities depicted in FIG. 2 occur due to irregularities in the directivity (intensity distribution) of the light-emitting element and the directivity (sensitivity distribution) of the light-receiving element.
The reflection characteristics of the reflecting reference member (shutter reference) are also difficult to manage with precision. For example, FIG. 2 shows measurements depicting the relationship between an LED current and sensor output when affixing two types of print film on the shutter as the reflecting reference member, where the vertical axis represents output from the density sensor and the horizontal axis represents the LED current (mA). Further, the thin solid line in FIG. 2 indicates the detection characteristics of the sensor when using a reflecting reference member A, and the bold line represents the detection characteristics when using a reflecting reference member B. From FIG. 2, we can see that the sensor output relative to the LED current varies according to which of the reflecting reference members A and B is used.
This difference in sensor output is a result of the amount of ink application or solvent dilution when printing the films. In order to keep the cost of parts low, it is preferable to be able to measure the toner quantity with stability, even when there is a slight irregularity in the reflection characteristics of the reflecting reference member.
It is an object of the present invention to provide an image-forming device which is capable of detecting toner density on an image-carrying member with high precision regardless of deviation or error in the image-forming device.